Predictors of permanent pacemaker implantation following transcatheter aortic valve replacement-the search is still on!

Several anatomical, demographic, clinical, electrocardiographic, procedural, and valve-related variables can be used to predict the probability of developing conduction abnormalities after transcatheter aortic valve replacement (TAVR) that necessitate permanent pacemaker (PPM) implantation. These variables include calcifications around the device landing zone and in the mitral annulus; pre-existing electrocardiographic abnormalities such as left and right bundle branch blocks (BBB), first- and second-degree atrioventricular blocks, as well as bifascicular and trifascicular blocks; male sex; diabetes mellitus (DM); hypertension; history of atrial fibrillation; renal failure; dementia; and use of self-expanding valves. The current study supports existing literature by demonstrating that type 2 DM and baseline right BBB are significant predictors of PPM implantation post-TAVR. Regardless of the side of the BBB, this study demonstrated, for the first time, a linear association between the incidence of PPM implantation post-TAVR and every 20 ms increase in baseline QRS duration (above 100 ms). After a 1-year follow-up, patients who received PPM post-TAVR had a higher rate of hospitalization for heart failure and nonfatal myocardial infarction.

Conduction system pacing and atrioventricular node ablation in heart failure: The PACE-FIB study design

AIMS

Atrial fibrillation (AF) worsens the prognosis of patients with heart failure (HF). Successful treatments are still very scarce for those with permanent AF and preserved (HFpEF) or mildly reduced (HFmrEF) ejection fraction. In this study, the long-term benefits and safety profile of heart rate regularization through left-bundle branch pacing (LBBP) and atrioventricular node ablation (AVNA) will be explored in comparison with pharmacological rate-control strategy.

METHODS AND RESULTS

The PACE-FIB trial is a multicentre, prospective, open-label, randomized (1:1) clinical study that will take place between March 2022 and February 2027. A total of 334 patients with HFpEF/HFmrEF and permanent AF will receive either LBBP followed by AVNA (intervention arm) or optimal pharmacological treatment for heart rate control according to European guideline recommendations (control arm). All patients will be followed up for a minimum of 36 months. The primary outcome measure will be the composite of all-cause mortality, HF hospitalization, and worsening HF at 36 months. Other secondary efficacy and safety outcome measures such as echocardiographic parameters, functional status, and treatment-related adverse events, among others, will be analysed too.

CONCLUSION

LBBP is a promising stimulation mode that may foster the clinical benefit of heart rate regularization through AV node ablation compared with pharmacological rate control. This is the first randomized trial specifically addressing the long-term efficacy and safety of this pace-and-ablate strategy in patients with HFpEF/HFmrEF and permanent AF.

Electrocardiography of Atrioventricular Block

Delayed atrioventricular (AV) conduction most commonly occurs in the AV node, resulting from AH prolongation on an intracardiac electrocardiogram and PR prolongation on a surface electrocardiogram. AV conduction may be blocked in a 2:1 manner, with a normal PR interval and wide QRS suggesting infranodal disease, whereas a prolonged PR interval and narrow QRS are more suggestive of AV nodal disease. Block within the His is suspected when there is 2:1 AV block with normal PR and QRS intervals. Complete heart block occurs when the atrial rhythm is totally independent of a junctional or lower escape rhythm.

Anatomy and Pathology of the Cardiac Conduction System

The cardiac conduction system is formed of histologically and electrophysiologically distinct specialized tissues uniquely located in the human heart. Understanding the anatomy and pathology of the cardiac conduction system is imperative to an interventional electrophysiologist to perform safe ablation and device therapy for the management of cardiac arrhythmias and heart failure. The current review summarizes the normal and developmental anatomy of the cardiac conduction system, its variation in the normal heart and congenital anomalies, and its pathology and discusses important clinical pearls for the proceduralist.

Genetic Abnormalities of the Sinoatrial Node and Atrioventricular Conduction

The peculiar electrophysiological properties of the sinoatrial node and the cardiac conduction system are key components of the normal physiology of cardiac impulse generation and propagation. Multiple genes and transcription factors and metabolic proteins are involved in their development and regulation. In this review, we have summarized the genetic underlying causes, key clinical findings, and the latest available clinical evidence. We will discuss clinical diagnosis and management of the genetic conditions associated with conduction disorders that are more prevalent in clinical practice, for this reason, very rare genetic diseases presenting sinus node or cardiac conduction system abnormalities are not discussed.

Systemic Diseases and Heart Block

Systemic diseases can cause heart block owing to the involvement of the myocardium and thereby the conduction system. Younger patients (<60) with heart block should be evaluated for an underlying systemic disease. These disorders are classified into infiltrative, rheumatologic, endocrine, and hereditary neuromuscular degenerative diseases. Cardiac amyloidosis owing to amyloid fibrils and cardiac sarcoidosis owing to noncaseating granulomas can infiltrate the conduction system leading to heart block. Accelerated atherosclerosis, vasculitis, myocarditis, and interstitial inflammation contribute to heart block in rheumatologic disorders. Myotonic, Becker, and Duchenne muscular dystrophies are neuromuscular diseases involving the myocardium skeletal muscles and can cause heart block.

Heinrich Ewald Hering's discovery of the heart pacemaker: Hering, Tawara and Aschoff's search for its morphological basis, the sinoatrial node, and why they failed

Two groups of investigators investigated the heart pacemaker and its morphological basis in the early twentieth century. The first group was formed by Henrich Ewald Hering (physiologist), Sunao Tawara and Ludwig Aschoff (morphologists). The second group was composed of James Mackenzie (general practitioner and clinical investigator), Arthur Keith and Martin Flack (morphologists). These groups were formed almost at the same time in 1903. Their work resulted in the discovery of the atrioventricular node and Purkinje network (Sunao Tawara, in 1906), heart pacemaker (H E Hering, in 1907) and sinoatrial node (Keith and Flack, in 1907). Here, it is shown how the interconnections of the concurrent works of these groups resulted in the discovery not only of the function, but also of the structure of the sinoatrial node.

Dual atrioventricular nodal non-reentrant tachycardia: Various atrioventricular conduction responses to atrioventricular simultaneous pacing

BACKGROUND

Specific pacing methods to unmask the existence of the dual atrioventricular (AV) nodal pathway in patients with dual AV nodal non-reentrant tachycardia remain to be established.

OBJECTIVE

This study aimed to determine the electrophysiological characteristics of dual AV nodal non-reentrant tachycardia by its responses to specific pacing methods.

METHODS

Five patients diagnosed as having dual AV nodal non-reentrant tachycardia were retrospectively investigated.

RESULTS

Atrial pacing could not induce the clinical tachycardia as continuous double firing in any of the 5 patients, but did induce sudden prolongation of the A-H interval as the linking phenomenon in 1 patient. A single atrial extrastimulation after sinus excitations was performed without interruption of double firing in 1 patient, and it induced the double ventricular response phenomenon within the limited range of the extrastimulus intervals. The pacing method of AV simultaneous basic pacing preceding atrial programmed extrastimulation did not allow interruptions of double firing during the basic drive trains and induced the double ventricular response phenomenon within the limited range of the extrastimulus intervals in all 5 patients, even in 1 patient without inducibility of the clinical tachycardia in the catheterization laboratory. The double ventricular response phenomenon within the limited range of the extrastimulus intervals may be based on the existence of the dual AV nodal pathway with concealed retrograde penetration.

CONCLUSION

The AV simultaneous basic pacing preceding atrial programmed extrastimulation method consistently and reproducibly unmasked the existence of the dual AV nodal pathway as the double ventricular response phenomenon in patients with dual AV nodal non-reentrant tachycardia.

Nonreentrant atrioventricular nodal tachycardia: The great simulator

We present the case of a healthy 12-year-old boy without structural heart disease who was followed for several years for an arrhythmia with diverse and incorrect initial diagnoses, refractory to flecainide, metoprolol and verapamil. Thorough reevaluation of the electrocardiographic recordings led to a presumptive diagnosis of a non-reentrant atrioventricular nodal tachycardia, subsequently confirmed with electrophysiology study. Radiofrequency ablation was performed during the procedure and the arrhythmia resolved. Double conduction through the atrioventricular node often simulates other arrhythmias, leading to misdiagnosis.

Miniseries 1-Part III: 'Behind the scenes' in the triangle of Koch

AIMS

To take full advantage of the knowledge of cardiac anatomy, structures should be considered in their correct attitudinal orientation. Our aim was to discuss the triangle of Koch in an attitudinally appropriate fashion.

METHODS AND RESULTS

We reviewed our material prepared by histological sectioning, along with computed tomographic datasets of human hearts. The triangle of Koch is the right atrial surface of the inferior pyramidal space, being bordered by the tendon of Todaro and the hinge of the septal leaflet of the tricuspid valve, with its base at the inferior cavotricuspid isthmus. The fibro-adipose tissues of the inferior pyramidal space separate the atrial wall from the crest of the muscular interventricular septum, thus producing an atrioventricular muscular sandwich. The overall area is better approached as a pyramid rather than a triangle. The apex of the inferior pyramidal space overlaps the infero-septal recess of the subaortic outflow tract, permitting the atrioventricular conduction axis to transition directly to the crest of the muscular ventricular septum. The compact atrioventricular node is formed at the apex of the pyramid by union of its inferior extensions, which represent the slow pathway, with the septal components formed in the buttress of the atrial septum, thus providing the fast pathway.

CONCLUSIONS

To understand its various implications in current cardiological catheter interventions, the triangle of Koch must be considered in conjunction with the inferior pyramidal space and the infero-septal recess. It is better to consider the overall region in terms of a pyramidal area of interest.

Miniseries 1-Part II: the comparative anatomy of the atrioventricular conduction axis

AIMS

The arrangement of the conduction axis is markedly different in various mammalian species. Knowledge of such variation may serve to question the validity of using animals as prospective models for design of systems for clinical use.

METHODS AND RESULTS

We compared the arrangement of the atrioventricular conduction axis in human, murine, canine, porcine, and bovine hearts, examining serially sectioned datasets from 20 human, 16 murine, 3 porcine, 5 canine, and 1 bovine hearts. We also analysed computed tomographic datasets obtained from bovines and one human heart. Unlike the situation in the human heart, there is no formation of an atrioventricular fibrous membranous septum in the murine, canine, porcine, nor bovine hearts. Canine, porcine, and bovine hearts also lack an infero-septal recess, when defined as a fibrous plate supporting the buttress of the atrial septum. In these species, half of the non-coronary leaflet is directly opposed to the ventricular septal surface.

CONCLUSION

There is a long right-sided non-branching component of the axis, which skirts the attachment of the non-coronary sinus of the aortic root. In the bovine heart, moreover, the left bundle branch usually extends intramyocardially as a solitary tape before surfacing and ramifying on the left ventricular septal surface. The difference in the atrioventricular conduction axis between species may influence the anatomical substrates for atrioventricular re-entry tachycardia, as well as providing inferences for assessing the risks of transcatheter implantation of the aortic valve. Further studies are now needed to assess these possibilities.

Electrocardiography of Atrioventricular Block

Delayed atrioventricular (AV) conduction most commonly occurs in the AV node, resulting from AH prolongation on an intracardiac electrocardiogram and PR prolongation on a surface electrocardiogram. AV conduction may be blocked in a 2:1 manner, with a normal PR interval and wide QRS suggesting infranodal disease, whereas a prolonged PR interval and narrow QRS are more suggestive of AV nodal disease. Block within the His is suspected when there is 2:1 AV block with normal PR and QRS intervals. Complete heart block occurs when the atrial rhythm is totally independent of a junctional or lower escape rhythm.

Characterization and functional role of rapid- and slow-activating delayed rectifier K+ currents in atrioventricular node cells of guinea pigs

The atrioventricular (AV) node is the only conduction pathway where electrical impulse can pass from atria to ventricles and exhibits spontaneous automaticity. This study examined the function of the rapid- and slow-activating delayed rectifier K⁺ currents (I_Kr and I_Ks) in the regulation of AV node automaticity. Isolated AV node cells from guinea pigs were current- and voltage-clamped to record the action potentials and the I_Kr and I_Ks current. The expression of I_Kr or I_Ks was confirmed in the AV node cells by immunocytochemistry, and the positive signals of both channels were localized mainly on the cell membrane. The basal spontaneous automaticity was equally reduced by E4031 and HMR-1556, selective blockers of I_Kr and I_Ks, respectively. The nonselective β-adrenoceptor agonist isoproterenol markedly increased the firing rate of action potentials. In the presence of isoproterenol, the firing rate of action potentials was more effectively reduced by the I_Ks inhibitor HMR-1556 than by the I_Kr inhibitor E4031. Both E4031 and HMR-1556 prolonged the action potential duration and depolarized the maximum diastolic potential under basal and β-adrenoceptor-stimulated conditions. I_Kr was not significantly influenced by β-adrenoceptor stimulation, but I_Ks was concentration-dependently enhanced by isoproterenol (EC₅₀: 15 nM), with a significant negative voltage shift in the channel activation. These findings suggest that both the I_Kr and I_Ks channels might exert similar effects on regulating the repolarization process of AV node action potentials under basal conditions; however, when the β-adrenoceptor is activated, I_Ks modulation may become more important.

Genetic Abnormalities of the Sinoatrial Node and Atrioventricular Conduction

The peculiar electrophysiological properties of the sinoatrial node and the cardiac conduction system are key components of the normal physiology of cardiac impulse generation and propagation. Multiple genes and transcription factors and metabolic proteins are involved in their development and regulation. In this review, we have summarized the genetic underlying causes, key clinical findings, and the latest available clinical evidence. We will discuss clinical diagnosis and management of the genetic conditions associated with conduction disorders that are more prevalent in clinical practice, for this reason, very rare genetic diseases presenting sinus node or cardiac conduction system abnormalities are not discussed.

Systemic Diseases and Heart Block

Systemic diseases can cause heart block owing to the involvement of the myocardium and thereby the conduction system. Younger patients (<60) with heart block should be evaluated for an underlying systemic disease. These disorders are classified into infiltrative, rheumatologic, endocrine, and hereditary neuromuscular degenerative diseases. Cardiac amyloidosis owing to amyloid fibrils and cardiac sarcoidosis owing to noncaseating granulomas can infiltrate the conduction system leading to heart block. Accelerated atherosclerosis, vasculitis, myocarditis, and interstitial inflammation contribute to heart block in rheumatologic disorders. Myotonic, Becker, and Duchenne muscular dystrophies are neuromuscular diseases involving the myocardium skeletal muscles and can cause heart block.

Anatomy and Pathology of the Cardiac Conduction System

The cardiac conduction system is formed of histologically and electrophysiologically distinct specialized tissues uniquely located in the human heart. Understanding the anatomy and pathology of the cardiac conduction system is imperative to an interventional electrophysiologist to perform safe ablation and device therapy for the management of cardiac arrhythmias and heart failure. The current review summarizes the normal and developmental anatomy of the cardiac conduction system, its variation in the normal heart and congenital anomalies, and its pathology and discusses important clinical pearls for the proceduralist.

Characteristics of patients with atrial flutter and spontaneous 1:1 atrioventricular conduction with and without anti-arrhythmic drug treatment

Atrial flutter (AFL) is a large reentrant circuit located in the right atrium. Anti-arrhythmic drugs (AADs) can provoke AFL with 1:1 atrioventricular conduction (AVC) to cause hemodynamic collapse. We elucidated the characteristics of patients with AFL exhibiting spontaneous 1:1 AVC. Fifteen patients (1:1 AFL group; 11 males, 52.4 ± 13.7 years old) who documented AFL with 1:1 AVC were enrolled and compared to 153 patients without 1:1 AVC (Control group; 137 males, 68.9 ± 11.2 years old). AFL cycle length during maximum AVC was significantly longer in the 1:1 AFL group than in the control group (274.7 ± 37.0 vs. 216.2 ± 25.6 ms, p < 0.001). Among 1:1 AVC group, 9 patients had AADs, and AFL cycle length was significantly longer during 1:1 AVC as compared with 2:1 AVC documented the other day (284.4 ± 41.3 vs. 233.3 ± 26.0 ms, p < 0.001), suggesting enhancement effect of the AADs during 1:1 AVC. Remaining 6 patients who did not take AADs, 2 patients showed enlargement of the tricuspid annulus and 3 patients developed 1:1 AVC during exercise. Multivariate analysis revealed that younger age and the use of AADs was independent risk factors for the development of 1:1 AFL group. Prolonged AFL cycle length associated with the class Ia/Ic AAD use, slower heart rate during sinus rhythm and younger age were important risk factors for the development of 1:1 AVC during AFL.

Genomic and physiological analyses of the zebrafish atrioventricular canal reveal molecular building blocks of the secondary pacemaker region

The atrioventricular canal (AVC) is the site where key structures responsible for functional division between heart regions are established, most importantly, the atrioventricular (AV) conduction system and cardiac valves. To elucidate the mechanism underlying AVC development and function, we utilized transgenic zebrafish line sqet31Et expressing EGFP in the AVC to isolate this cell population and profile its transcriptome at 48 and 72 hpf. The zebrafish AVC transcriptome exhibits hallmarks of mammalian AV node, including the expression of genes implicated in its development and those encoding connexins forming low conductance gap junctions. Transcriptome analysis uncovered protein-coding and noncoding transcripts enriched in AVC, which have not been previously associated with this structure, as well as dynamic expression of epithelial-to-mesenchymal transition markers and components of TGF-β, Notch, and Wnt signaling pathways likely reflecting ongoing AVC and valve development. Using transgenic line Tg(myl7:mermaid) encoding voltage-sensitive fluorescent protein, we show that abolishing the pacemaker-containing sinoatrial ring (SAR) through Isl1 loss of function resulted in spontaneous activation in the AVC region, suggesting that it possesses inherent automaticity although insufficient to replace the SAR. The SAR and AVC transcriptomes express partially overlapping species of ion channels and gap junction proteins, reflecting their distinct roles. Besides identifying conserved aspects between zebrafish and mammalian conduction systems, our results established molecular hallmarks of the developing AVC which underlies its role in structural and electrophysiological separation between heart chambers. This data constitutes a valuable resource for studying AVC development and function, and identification of novel candidate genes implicated in these processes.

Ablation of Accessory Pathways with Challenging Anatomy

Although catheter ablation of accessory pathways is deemed highly safe and effective, peculiar location of these pathways might lead to complex and potentially hazardous procedures requiring ablation in anatomic regions such as para-Hisian area, coronary sinus, and epicardial surface. The electrophysiologist should know these possible scenarios to plan the best strategy for safe and effective ablation of these uncommon accessory pathways.

Morphological variations of the conduction system in the atrioventricular zone and its clinical relationship in different species

Atrioventricular node is responsible for delaying the passage of the electrical impulse to ventricles in order to protect them from fast depolarizations coming from the atria. The importance of this study is to identify the morphological variations of the components of atrioventricular zone that affect the conduction system and its clinical relationship in different species of mammals. We analyzed ten human hearts, nine from horses, eight from pigs, and five from dogs without a clinical history of cardiac pathologies. Histological section thickness of 5 μm were obtained with a microtome and stained with hematoxylin-eosin and Masson's trichrome. We observed both an increase in collagen fibers and a decrease in the size of P cells (nodal pacemaker cells) within the atrioventricular node in dogs, horses and pigs in cases that presented cartilage in fibrous body. The percentage of fundamental substance in atrioventricular node was significantly higher in dogs and the percentage of collagen fibers was higher in pigs, both than in humans. The presence of cartilaginous metaplasia in cardiac fibrous skeleton from different species decreases the size of atrioventricular node and its cells and increases the percentage of collagen fibers within the node, which can reduce the transmission of the electrical impulse to ventricles and therefore predispose to the presentation of ventricular arrhythmias. Morphometric analysis has allowed us to objectively quantify each of the components of AV node and compare them in the different species.

Imaging of Atrioventricular Nodal Conduction Tissue in Porcine Hearts Using Optical Coherence Tomography

Optical coherence tomography (OCT) employs near-infrared light to image the microstructure of different tissues. Clinically, it has been used to image the walls of coronary arteries. In research settings, one of the applications for OCT is visualizing endocardial and subendocardial structures. The present experiment sought to determine whether OCT can identify native conduction tissues in adult porcine hearts. During the study, the right atrial endocardial surfaces of excised adult porcine hearts were exposed. The triangle of Koch was imaged with the OCT system and the conduction tissue was identified. The area was then prepared for histologic examination with Masson's trichrome stain. The results of histologic preparations and OCT images were then compared. Ultimately, nine porcine hearts were examined using this methodology. OCT imaging successfully identified subendocardial structures presumed to be the compact atrioventricular node. Histologic images of the preparations delineated the different tissue types and conduction tissue was easily identified. The location of distinctive hyporeflective areas in the OCT images correlated with the location of conduction tissue in the histology images. In light of the findings of this study, it is suggested that atrioventricular nodal tissue can be identified by OCT in freshly dissected unfixed porcine hearts. OCT images distinguished the differentiated conduction tissue in close proximity with the endocardium, myofibers, and fibrous tissue, and the success of this was verified with histology. This technology may be useful for the direct visualization of the native conduction system during procedures in the operating room and electrophysiology laboratory. Further studies with perfused tissue samples and live animal experiments are needed to better assess the efficacy of this novel application.

Atrio-ventricular junction: Can precision electrocardiology bridge cell and electrocardiogram?

The Atrio Ventricular Junction (AVJ) is a well-defined anatomical region of the heart the physiology of which, despite extensive and numerous observations, it is not fully understood. The aim of this review is to present an up to date summary of old and more recent findings on histology, cellular electrophysiology and intracellular connectivity of this region. We have also attempted to relate our increasing understanding of nodal pathophysiology to the interpretation of the electrocardiographic (ECG) manifestations of AVN behavior. Bridging cellular observations with ECG analysis in a process we call "Precision Electrocardiology" renders this tool far more sensitive and clinically useful than the pattern analysis too often employed in the ECG interpretation.

Sunao Tawara : further musings on his tribulations in providing the basis for the modern-day understanding of cardiac electrophysiology

Sunao Tawara, who was born in 1873 and died in 1952, is considered the father of modern cardiac electrophysiology. He published his monumental monograph describing the atrioventricular conduction axis in 1906. He achieved this task in the face of multiple tribulations as a doctoral student working in a cultural environment that was not his own. Although his letters underscoring the publication of the monograph have been published, little emphasis has been placed on the potential problems he encountered in bringing his task to fruition. For example, it was not until the final 6 months of his studies that he resolved the issue of the connection between the atrioventricular bundle and the so called "Purkinje cardiomyocytes". His exchanges with his mentor, Ludwig Aschoff, emphasized that the difficulties he encountered in making the connection caused him quite some turmoil. We believe that this issue, and others that he identified in his correspondence, are worthy of further attention.

Emerging Function of Cardiac Macrophages Ushers in a New Era for the Electrophysiology of the Heart

Maintaining a coordinated heart rhythm is essential for maintaining the heart’s pumping function and blood circulation. Every heartbeat is generated by electrical impulse propagation that is passing through gap junctions, which are composed of connexin proteins. In mammalian hearts, Cx43, Cx40, Cx45, and Cx30.2 are expressed and regulated by post-translational modification. Cardiac macrophages account for only a small number of total heart cells, but they reside all around the heart. They are primarily established prenatally, and they arise from embryonic yolk sac progenitors. Recently, increasing attention has been directed toward novel roles for cardiac resident macrophages, especially in the heart’s electrical impulse conduction. Here, we provide an overview of the recent findings on connexins, with a focus on the emerging function of cardiac macrophages, and we discuss the future directions of treatment for heart disease.

Ivabradine for chronic heart rate control in persistent atrial fibrillation. Design of the BRAKE-AF project

INTRODUCTION AND OBJECTIVES

Ivabradine is an inhibitor of the If channel, the main determinant of the pacemaker function of the sinus node. The drug has been approved for the treatment of angina and heart failure. There is some evidence of its role as an inhibitor of atrial-ventricular node (AVN) conduction. The aim of the BRAKE-AF project is to assess ivabradine use for rate control in atrial fibrillation (AF).

METHODS

A multicenter, randomized, parallel, open-label, noninferiority phase III clinical trial will be conducted to compare ivabradine vs digoxin in 232 patients with uncontrolled permanent AF despite beta-blockers or calcium channel blockers. The primary efficacy endpoint is the reduction in daytime heart rate measured by 24-hour Holter monitoring at 3 months. This clinical trial will be supported by an electrophysiological study of the effect of ivabradine on the action potential of the human AVN. To do this, an experimental model will be used with Chinese hamster ovarium cells transfected with the DNA encoding the expression of the t channels involved in this action potential and recording of the ionic currents with patch clamp techniques.

RESULTS

New data will be obtained on the effect of ivabradine on the human AVN and its safety and efficacy in patients with permanent AF.

CONCLUSIONS

The results of the BRAKE-AF project might allow inclusion of ivabradine within the limited arsenal of drugs currently available for rate control in AF.

CLINICAL TRIAL REGISTRATION

http://www.clinicaltrials.gov. Identifier: NCT03718273.

Safety and Efficacy Outcomes of Combined Leadless Pacemaker and Atrioventricular Nodal Ablation for Atrial Fibrillation Using a Single Femoral Puncture Approach

BACKGROUND

Atrioventricular nodal (AVN) ablation with permanent pacemaker (PPM) insertion is indicated for rate control in patients with atrial fibrillation (AF) who remain unresponsive to rate or rhythm control strategies. The leadless PPM (Micra Transcatheter Pacing System [TPS], Medtronic, Minneapolis, MN, USA) has the advantage of eliminating transvenous lead and pacemaker pocket-related complications. The aim of this case series was to determine the outcomes of patients who had undergone combined Micra TPS and AVN ablation, performed via a single femoral approach.

METHOD

A retrospective review was undertaken on patients who had undergone concurrent procedures, across two major hospitals in Perth, Western Australia. Procedural details were obtained from a cardiac devices database whilst patient demographics and clinical information were determined from medical records.

RESULTS

Fourteen (14) patients underwent concurrent Micra TPS insertion and AVN ablation for symptomatic AF. The average age was 73±9.2 years, and 43% of them were males. There was no acute procedural/device related complication. Over a median follow-up duration of 9 months (36% completing 12-month follow-up), there was no incidence of device complications, in particular device dislodgement, malfunction or infection. One patient had a resuscitated ventricular fibrillation (VF) arrest event with new onset cardiomyopathy during follow-up and required Micra TPS removal. One patient died at 33 days post procedure from a non-cardiac cause. Device performance was excellent with stable sensing and pacing thresholds during the follow-up period.

CONCLUSION

Our study has shown that combined leadless PPM (Micra TPS) implantation and AVN ablation using a single femoral approach is feasible, with good safety and efficacy profile in the short-medium term. Long-term data involving larger cohorts is needed to confirm the findings of this study and determine the clinical usefulness of this combined approach.

Cardiovascular Collapse with Intravenous Amiodarone in Children: A Multi-Center Retrospective Cohort Study

OBJECTIVE

To determine the incidence of cardiovascular collapse in children receiving intravenous (IV) amiodarone and to identify the population at risk.

DESIGN

A multicenter study of patients ≤ 18 years of age who received intravenous amiodarone between January 2005 and December 2015. A retrospective analysis was performed to identify patients who developed cardiovascular collapse (bradycardia and/or hypotension).

RESULTS

Of 456 patients who received amiodarone, cardiovascular collapse occurred in 47 patients (10%). Patient risk factors for collapse in a univariate analysis were as follows: age < 3 months (p = 0.04), depressed cardiac function (p < 0.001), blood pressure below 3rd percentile (p < 0.001), high lactate at baseline (p < 0.001). Administration risk factors included bolus administration (p = 0.04), and bolus administration over ≤ 20 min (p = 0.04). In multivariate analysis, age, baseline blood pressure less than 3rd percentile, and rapid bolus delivery were independent risk factors for cardiovascular collapse in the study group. The mortality rate was significantly higher in the collapse group (28% versus 8%).

CONCLUSION

We found an association between IV amiodarone administration and the risk of developing cardiovascular collapse in a significant subset of children. Extreme caution and careful hemodynamic monitoring is recommended when using IV amiodarone in this population, especially in young infants, hemodynamically compromised patients, and in patients receiving rapid amiodarone bolus administration.

Direct His-bundle Pacing in a Patient with a Persistent Left Superior Vena Cava

We report a case of direct His-bundle lead placement at the time of implantable cardioverter-defibrillator insertion and atrioventricular node ablation. The patient was found to have an isolated persistent left superior vena cava, and selective His-bundle pacing was successfully achieved through the use of a steerable sheath and dedicated mapping catheter.

Field dynamics in atrioventricular activation. Clinical evidence of a specific field-to-protein interaction

The atrioventricular node (AV) is considered the electrical connection between the atria and ventricles. There is an electrical pause between activation of the atria and the ventricles (PR segment), but to date the mechanism responsible for this interruption remains unclear. The present communication focuses on the hypothesis that magnetic field dynamics could provide the answer. Proof of this hypothesis is that in Wolff-Parkinson-White syndrome, where there is physical connection between the atria and ventricles (bundle of Kent), there is electrical AV continuity, no PR segment is detected, and catheter ablation of the abnormal bundle restores AV discontinuity. Spontaneous initiation of the heart at the level of the sinus node, the pacemaker of the heart, could also be explained via field dynamics. The known transmembrane pacemaker protein CHN4, present in both sinoatrial and AV nodal cells, could interact with field information to provide specificity in an electronic key-to-lock mechanism interaction.

Advanced Concepts of Atrioventricular Nodal Electrophysiology: Observations on the Mechanisms of Atrioventricular Nodal Reciprocating Tachycardias

Atrioventricular node reentrant tachycardia (AVNRT) is a supraventricular arrhythmia easily diagnosed by 12-lead electrocardiogram. What is far more challenging, is the understanding of the reentrant circuit in its typical and atypical presentations. The function of the atrioventricular node is still incomplete and this knowledge gap is reflected in the reconstruction of the pathways used by AVNRT in its multiform presentations. This article illustrates the heterogeneous electrocardiographic manifestations of AVNRT. We reconstruct the reentrant circuits involved using more recent understanding of the anatomic and electrophysiologic characteristics of the atrioventricular node.

Embryonic Tbx3+ cardiomyocytes form the mature cardiac conduction system by progressive fate restriction

A small network of spontaneously active Tbx3+ cardiomyocytes forms the cardiac conduction system (CCS) in adults. Understanding the origin and mechanism of development of the CCS network are important steps towards disease modeling and the development of biological pacemakers to treat arrhythmias. We found that Tbx3 expression in the embryonic mouse heart is associated with automaticity. Genetic inducible fate mapping revealed that Tbx3+ cells in the early heart tube are fated to form the definitive CCS components, except the Purkinje fiber network. At mid-fetal stages, contribution of Tbx3+ cells was restricted to the definitive CCS. We identified a Tbx3+ population in the outflow tract of the early heart tube that formed the atrioventricular bundle. Whereas Tbx3+ cardiomyocytes also contributed to the adjacent Gja5+ atrial and ventricular chamber myocardium, embryonic Gja5+ chamber cardiomyocytes did not contribute to the Tbx3+ sinus node or to atrioventricular ring bundles. In conclusion, the CCS is established by progressive fate restriction of a Tbx3+ cell population in the early developing heart, which implicates Tbx3 as a useful tool for developing strategies to study and treat CCS diseases.

Low-energy ablation of anteroseptal accessory pathways in two dogs

In humans, accessory pathways (APs) in an anteroseptal and midseptal position are often challenging to ablate because of their close proximity with the conduction pathways of the atrioventricular junction. The use of low-energy ablation techniques can be useful to reduce the risk of permanently damaging the atrioventricular node and the His bundle. This report describes the use of low-energy radiofrequency catheter ablation to successfully and permanently ablate anteroseptal APs in two dogs with orthodromic atrioventricular reciprocating tachycardia. In the first dog, a transient first degree atrioventricular block persisted for 30 s after radiofrequency energy delivery. In the second dog, transient paroxysmal atrioventricular conduction block was observed during the procedure but resolved within 3 days. First degree atrioventricular block was again identified 2 months later. In conclusion, anteroseptal APs can be effectively treated by low-energy radiofrequency catheter ablation with minimal and transient damage to the atrioventricular junction.

Adenosine hypersensitivity and atrioventricular block

Adenosine is a ubiquitous substance that is released under several physiological and pathological conditions and has cardiovascular effects including cardioinhibition and vasodilation. It has been shown to be an important modulator implicated in several forms of syncope. In patients with chronic low plasma levels of adenosine, a transient release of endogenous adenosine can be sufficient to block conduction in the atrioventricular node and induce prolonged asystole; conversely, when plasma adenosine levels are chronically high, adenosine release is responsible for vasodepression. Distinct purinergic profiles in patients presenting with syncope have recently been correlated with the clinical presentation: "low-adenosine patients," prone to asystole, may present with idiopathic atrioventricular block, carotid sinus syndrome, or syncope with no or very brief prodromes and normal heart; "high-adenosine patients," prone to vasodilation, experience vasovagal syncope. This pathophysiological classification may have therapeutic implications.

Junctional Rhythm Preoperatively and During General Anesthesia for Oral and Maxillofacial Surgery

We report a case of junctional rhythm that occurred both preoperatively and later during a portion of general anesthesia. A 19-year-old woman was scheduled to undergo bilateral sagittal split ramus osteotomy after being diagnosed with a jaw deformity. Preoperative electrocardiography (ECG) revealed a junctional rhythm with a slow heart rate (HR). At 90 minutes after anesthesia induction, local anesthesia with 10 mL of 1% lidocaine and 1:100,000 adrenaline was administered. A junctional rhythm appeared 15 minutes after the local anesthesia. We believe that the atrioventricular nodal pacemaker cells accelerated because of the increased sympathetic activity due to the adrenaline. On the preoperative ECG, the junctional rhythm with slow HR appeared as an escaped beat caused by slowing of the primary pacemaker. Therefore, we think that the preoperative junctional rhythm and the junctional rhythm that appeared during general anesthesia were due to different causes. Understanding the cause of a junctional rhythm could lead to more appropriate treatment. We therefore believe that identifying the cause of the junctional rhythm is important in anesthetic management.

Histological topography of the atrioventricular node and its extensions in relation to the cardiothoracic surgical landmarks in normal human hearts

BACKGROUND

Atrioventricular (AV) nodal injury which results in cardiac conduction disorders is one of the potential complications of heart valve surgeries and radiofrequency catheter ablations. Understanding the topography of the AV conduction system in relation to the tricuspid and mitral valves will help in reducing these complications.

METHODS

A tissue block of 3cmx4cm, which contain the AV node, bundle of His and the AV nodal extensions, was excised at the AV septal junction in 20 apparently normal human hearts. The block was divided into three equal segments through vertical incisions perpendicular to the insertion of the septal leaflet of the tricuspid valve. Each segment was processed and stained with H&E and Gomori to study the different parts of the AV conduction system.

RESULTS

The lower pole of the AV node was located vertically above the tricuspid septal leaflet (TSL) in 100% (20/20) of cases and at the level of the muscular interventricular septum in 65% (13/20) of cases. The upper pole of the compact AV node was located at the level of the mitral valve leaflet (MVL) in 50% (10/20) of cases. The penetrating bundle of His was seen at the level of the TSL, while the branching bundle of His was situated 1.9±1.5 mm inferior to the TSL. The right and left posterior extensions of the AV node spanned from the MVL to 2.9±1.3 mm above the TSL.

CONCLUSIONS

A rectangular area (2.5 mm × 12 mm) in the Koch's triangle was devoid of AV nodal tissue and could be labeled as a safe area with no risk of conduction defects during valve surgeries. Information on the separation of AV nodal extensions from the TSL, MVL and muscular interventricular septum may play a crucial role in guiding and improving the safety of radiofrequency ablations.

Increased sodium/calcium exchanger activity enhances beta-adrenergic-mediated increase in heart rate: Whole-heart study in a homozygous sodium/calcium exchanger overexpressor mouse model

BACKGROUND

The cardiac sodium/calcium (Na⁺/Ca²⁺) exchanger (NCX) contributes to diastolic depolarization in cardiac pacemaker cells. Increased NCX activity has been found in heart failure and atrial fibrillation. The influence of increased NCX activity on resting heart rate, beta-adrenergic-mediated increase in heart rate, and cardiac conduction properties is unknown.

OBJECTIVE

The purpose of this study was to investigate the influence of NCX overexpression in a homozygous transgenic whole-heart mouse model (NCX-OE) on sinus and AV nodal function.

METHODS

Langendorff-perfused, beating whole hearts of NCX-OE and the corresponding wild-type (WT) were studied ± isoproterenol (ISO; 0.2 μM). Epicardial ECG, AV nodal Wenckebach cycle length (AVN-WCL), and retrograde AVN-WCL were obtained.

RESULTS

At baseline, basal heart rate was unaltered between NCX-OE and WT (WT: cycle length [CL] 177.6 ± 40.0 ms, no. of hearts [n] = 20; NCX-OE: CL 185.9 ± 30.5 ms, n = 18; P = .21). In the presence of ISO, NCX-OE exhibited a significantly higher heart rate compared to WT (WT: CL 133.4 ± 13.4 ms, n = 20; NCX-OE: CL 117.7 ± 14.2 ms, n = 18; P <.001). ISO led to a significant shortening of the anterograde and retrograde AVN-WCL without differences between NCX-OE and WT.

CONCLUSION

This study is the first to demonstrate that increased NCX activity enhances beta-adrenergic increase of heart rate. Mechanistically, increased NCX inward mode activity may promote acceleration of diastolic depolarization in sinus nodal pacemaker cells, thus enhancing chronotropy in NCX-OE. These findings suggest a novel potential therapeutic target for heart rate control in the presence of increased NCX activity, such as heart failure.

Geometry of Koch's triangle

Aims

The first aim of this study was to determine the size of the Koch's triangle. The second one was to investigate relation between its dimensions and other individual-specific and heart-specific parameters as well as to create universal formula to estimate triangle dimensions based on these parameters.

Methods and results

This study is a prospective one, presenting 120 randomly selected autopsied hearts dissected from adult humans (Caucasian) of both sexes (31.7% females), with mean age of 49.3 ± 17.4 years. The length of triangle sides and angles were measured and the triangle area was calculated as well. Sixteen additional heart parameters were measured in order to analyse potential relationship between the dimensions of Koch's triangle and other dimensions of the heart, using linear regression analysis. The mean (±SD) length of the anterior edge was approximated to 18.0 ± 3.8 mm, the posterior edge to 20.3 ± 4.3 mm, and the basal edge to 18.5 ± 4.0 mm. The average values of the apex angle, the Eustachian angle, and the septal leaflet angle were 58.0 ± 14.4°, 53.8 ± 10.6°, and 67.6 ± 14.4°, respectively. The mean value of the Koch's triangle area was 151.5 ± 55.8 mm2. The 95th percentile of triangle's height (the distance from the apex to the coronary sinus) was 21.8 mm.

Conclusion

Mean values and proportions of triangle's sides and angles were presented. Koch's triangle showed considerable individual variations in size. The dimensions of the triangle were strongly independent from individual-specific and heart-specific morphometric parameters; however, the maximum triangle's height can be estimated as 22 mm.

Atrioventricular Junction Ablation for Atrial Fibrillation

Atrioventricular junction (AVJ) ablation is an effective therapy in patients with symptomatic atrial fibrillation who are intolerant to or unsuccessfully managed with rhythm control or medical rate control strategies. A drawback is that the procedure mandates a pacing system. Overall, the safety and efficacy of AVJ ablation is high with a majority of the patients reporting significant improvement in symptoms and quality-of-life measures. Risk of sudden cardiac death after device implantation is low, especially with an appropriate postprocedure pacing rate. Mortality benefit with AVJ ablation has been shown in patients with heart failure and cardiac resynchronization therapy devices.

Hyperhomocysteinemia Alters Sinoatrial and Atrioventricular Nodal Function: Role of Magnesium in Attenuating These Effects

Patients with hyperhomocysteinemia (HHcy), or elevated plasma homocysteine (Hcy), are at higher risk of developing arrhythmias and sudden cardiac death; however, the mechanisms are unknown. In this study, the effects of HHcy on sinus node function, atrioventricular conduction, and ventricular vulnerability were investigated by electrophysiological (EP) analysis, and the role of magnesium (Mg(2+)), an endogenous N-methyl-D-aspartate (NMDA) receptor antagonist, in attenuating EP changes due to HHcy was explored. Wild-type mice (WT) and mice receiving Hcy in the drinking water for 12 weeks (DW) were subjected to electrocardiographic and EP studies. DW compared to WT had significantly shorter RR, PR, QT, and HV intervals, corrected sinus node recovery times (CSNRT), Wenckebach periodicity (WP), atrioventricular nodal effective refractory periods (AVNERP), and right ventricular effective refractory periods (RVERP). To examine the role of Mg(2+) in mitigating conduction changes in HHcy, WT, DW, and heterozygous cystathionine-β-synthase knockout mice (CBS (+/-) ) were subjected to repeat EP studies before and after administration of low-dose magnesium sulfate (20 mg/kg). Mg(2+) had no effect on EP variables in WT, but significantly slowed CSNRT, WP, and AVNERP in DW, as well as WP and AVNERP in CBS (+/-) . These findings suggest that ionic channels modulated by Mg(2+) may contribute to HHcy-induced conduction abnormalities.

Multiple ion channel block by the cation channel inhibitor SKF-96365 in myocytes from the rabbit atrioventricular node

The atrioventricular node (AVN) of the cardiac conduction system coordinates atrial-ventricular excitation and can act as a subsidiary pacemaker. Recent evidence suggests that an inward background sodium current, IB,Na, carried by nonselective cation channels (NSCCs), contributes to AVN cell pacemaking. The study of the physiological contribution of IB,Na has been hampered, however, by a lack of selective pharmacological antagonists. This study investigated effects of the NSCC inhibitor SKF-96365 on spontaneous activity, IB,Na, and other ionic currents in AVN cells isolated from the rabbit. Whole-cell patch-clamp recordings of action potentials (APs) and ionic currents were made at 35-37°C. A concentration of 10 μmol/L SKF-96365 slowed spontaneous action potential rate by 13.9 ± 5.3% (n = 8) and slope of the diastolic depolarization from 158.1 ± 30.5 to 86.8 ± 30.5 mV sec(-1) (P < 0.01; n = 8). Action potential upstroke velocity and maximum diastolic potential were also reduced. Under IB,Na-selective conditions, 10 μmol/L SKF-96365 inhibited IB,Na at -50 mV by 36.1 ± 6.8% (n = 8); however, effects on additional channel currents were also observed. Thus, the peak l-type calcium current (ICa,L) at +10 mV was inhibited by 38.6 ± 8.1% (n = 8), while the rapid delayed rectifier current, IKr, tails at -40 mV following depolarization to +20 mV were inhibited by 55.6 ± 4.6% (n = 8). The hyperpolarization-activated current, If, was unaffected by SKF-96365. Collectively, these results indicate that SKF-96365 exerts a moderate inhibitory effect on IB,Na and slows AVN cell pacemaking. However, additional effects of the compound on ICa,L and IKr confound the use of SKF-96365 to dissect out selectively the physiological role of IB,Na in the AVN.

Ischemia-reperfusion destabilizes rhythmicity in immature atrioventricular pacemakers: A predisposing factor for postoperative arrhythmias in neonate rabbits

BACKGROUND

Postoperative arrhythmias such as junctional ectopic tachycardia and atrioventricular block are serious postoperative complications for children with congenital heart disease. We hypothesize that ischemia-reperfusion (I/R) related changes exacerbate these postoperative arrhythmias in the neonate heart and administration of postoperative inotropes is contributory.

OBJECTIVE

The purpose of this study was to study the effects of I/R and postischemic dopamine application on automaticity and rhythmicity in immature and mature pacemaker cells and whole heart preparations.

METHODS

Single pacemaker cells and whole heart models of postoperative arrhythmias were generated in a rabbit model encompassing 3 primary risk factors: age, I/R exposure, and dopamine application. Single cells were studied using current clamp and line scan confocal microscopy, whereas whole hearts were studied using optical mapping.

RESULTS

Four responses were observed in neonatal atrioventricular nodal cells (AVNCs): slowing of AVNC automaticity (from 62±10 to 36 ± 12 action potentials per minute, P<.05); induction of arrhythmicity or increased beat-to-beat variability (0.08 ± 0.04 to 3.83 ± 1.79, P<.05); altered automaticity (subthreshold electrical fluctuations); and disruption of calcium transients. In contrast, these responses were not observed in mature AVNCs or neonatal sinoatrial cells. In whole heart experiments, neonatal hearts experienced persistent postischemia arrhythmias of varying severity, whereas mature hearts exhibited no arrhythmias or relatively transient ones.

CONCLUSION

Neonatal pacemaker cells and whole hearts demonstrate a susceptibility to I/R insults resulting in alterations in automaticity, which may predispose neonates to postoperative arrhythmias such as junctional ectopic tachycardia and atrioventricular block.

Characterization and influence of cardiac background sodium current in the atrioventricular node

Background inward sodium current (IB,Na) that influences cardiac pacemaking has been comparatively under-investigated. The aim of this study was to determine for the first time the properties and role of IB,Na in cells from the heart's secondary pacemaker, the atrioventricular node (AVN). Myocytes were isolated from the AVN of adult male rabbits and mice using mechanical and enzymatic dispersion. Background current was measured using whole-cell patch clamp and monovalent ion substitution with major voltage- and time-dependent conductances inhibited. In the absence of a selective pharmacological inhibitor of IB,Na, computer modelling was used to assess the physiological contribution of IB,Na. Net background current during voltage ramps was linear, reversing close to 0mV. Switching between Tris- and Na(+)-containing extracellular solution in rabbit and mouse AVN cells revealed an inward IB,Na, with an increase in slope conductance in rabbit cells at -50mV from 0.54±0.03 to 0.91±0.05nS (mean±SEM; n=61 cells). IB,Na magnitude varied in proportion to [Na(+)]o. Other monovalent cations could substitute for Na(+) (Rb(+)>K(+)>Cs(+)>Na(+)>Li(+)). The single-channel conductance with Na(+) as charge carrier estimated from noise-analysis was 3.2±1.2pS (n=6). Ni(2+) (10mM), Gd(3+) (100μM), ruthenium red (100μM), or amiloride (1mM) produced modest reductions in IB,Na. Flufenamic acid was without significant effect, whilst La(3+) (100μM) or extracellular acidosis (pH6.3) inhibited the current by >60%. Under the conditions of our AVN cell simulations, removal of IB,Na arrested spontaneous activity and, in a simulated 1D-strand, reduced conduction velocity by ~20%. IB,Na is carried by distinct low conductance monovalent non-selective cation channels and can influence AVN spontaneous activity and conduction.

His electrogram alternans (Zhang's phenomenon) and a new model of dual pathway atrioventricular node conduction

BACKGROUND

In contrast to the current textbook model and the current clinical index of dual pathway atrioventricular (AV) nodal conduction, here we summarize the discovery and validation of Zhang's phenomenon (originally His electrogram alternans) as a new index of dual pathway conduction. We also describe the new findings of transverse-versus-longitudinal electrical propagation within the AV node as the electrophysiological basis underlining this new index. Thus, a new index and a new model of dual pathway AV conduction are being developed.

METHODS

We have reviewed current literature and provided evidence supporting a new index and a new model of dual pathway AV conduction.

RESULTS

Recent data revealed that during fast pathway conduction, electrical excitation in the AV node propagates in a superior to inferior direction across AV conduction axis and fiber orientation to reach first the superior His bundle fibers. However, this transverse conduction can fail easily within the superior nodal domain at fast rates. The failing of transverse propagation permits electrical excitation formed at the posterior/inferior nodal region to propagate longitudinally along fiber orientation in a posterior to anterior direction through the inferior nodal domain to reach the inferior His bundle (slow pathway conduction). This transverse-versus-longitudinal electrical propagation within the AV node results in a functional dissociation in the distal node and formation of dual inputs into the His bundle, providing the electrophysiological basis for the formation of Zhang's phenomenon (His electrogram alternans).

CONCLUSIONS

Based on strong experimental data, a new index and a new model of dual pathway AV nodal conduction are emerging, although they are still awaiting clinical validation.

Characterization of L-type calcium channel activity in atrioventricular nodal myocytes from rats with streptozotocin-induced Diabetes mellitus

Cardiovascular complications are common in patients with Diabetes mellitus (DM). In addition to changes in cardiac muscle inotropy, electrical abnormalities are also commonly observed in these patients. We have previously shown that spontaneous cellular electrical activity is altered in atrioventricular nodal (AVN) myocytes, isolated from the streptozotocin (STZ) rat model of type-1 DM. In this study, utilizing the same model, we have characterized the changes in L-type calcium channel activity in single AVN myocytes. Ionic currents were recorded from AVN myocytes isolated from the hearts of control rats and from those with STZ-induced diabetes. Patch-clamp recordings were used to assess the changes in cellular electrical activity in individual myocytes. Type-1 DM significantly altered the cellular characteristics of L-type calcium current. A reduction in peak I_CaL density was observed, with no corresponding changes in the activation parameters of the current. L-type calcium channel current also exhibited faster time-dependent inactivation in AVN myocytes from diabetic rats. A negative shift in the voltage dependence of inactivation was also evident, and a slowing of restitution parameters. These findings demonstrate that experimentally induced type-1 DM significantly alters AVN L-type calcium channel cellular electrophysiology. These changes in ion channel activity may contribute to the abnormalities in cardiac electrical function that are associated with high mortality levels in patients with DM.